1. Field of the Invention
The present invention relates to an integrated transformer structure and, more particularly, to an integrated transformer which can be formed on a single silicon chip and is capable of providing high-voltage isolation between input and output signals.
2. Description of the Prior Art
In the current world of semiconductor electronics, the need for ever-smaller components is constantly increasing. Unlike many other standard electrical components, the miniaturization of the transformer has proven to be a difficult task. For example, conventional wound transformers are difficult to make in small sizes, especially since the windings are typically made of very tiny wire. A miniaturized transformer of this structure is disclosed in U.S. Pat. No. 4,103,267 issued to W. W. Olschewski on July 25, 1978. In this device, a ceramic substrate is provided with a plurality of planar conductors which extend radially from an imaginary point on the surface of the substrate. A layer of dielectric material is formed over a major portion of each of the conductors to form a ring of dielectric material to which a ferrite toroidal core is adhesively secured. The core is precoated with an insulating material prior to being secured to the dielectric ring. A plurality of wire conductors are wire bonded at each end thereof to the exposed ends of the metal conductors on the substrate. Although this structure is significantly smaller than a conventional ferrite core transformer, its use is limited to hybrid component manufacture, and as disclosed cannot be totally integrated into the ceramic substrate.
An improvement over the Olschewski arrangement is disclosed in U.S. Pat. No. 4,547,961 issued to D. R. Bokil et al on Oct. 22, 1985. The Bokil et al patent discloses a miniaturized thick-film isolation transformer. In particular, the arrangement includes two rectangular substrates, each carrying successive screen-printed thick-film layers of dielectric with embedded spiral planar windings. The substrates and the dielectric layers are formed with a central opening in which is positioned the central leg of a three-legged solid magnetic core. The remaining portions of the core surround the two substrates. Although this arrangement is relatively small in size (0.75".times.0.55" with a height of 0.2"), it is still too large to be formed on a silicon substate and integrated with other components.
The problem remaining in the prior art, therefore, is to provide a completely integrated transformer capable of being formed on a single silicon substrate which is able to provide the isolation required for high voltage integrated circuit applications.